Dust storms can greatly alter the opacity (τ) on Mars. While (up through MSL Sol 1828) the REMS Team lists all sols at MSL with opacity as “sunny,” this claim is directly contracted by the Malin Space Science Systems (MSSS) in their weekly Martian weather reports at http://www.msss.com/msss_images/subject/weather_reports.html. Figure 36 shows REMS daily reports labelled SUNNY although the Malin reports raise doubts. Table 14 shows 38 weeks of weather reports by MSSS that seem to contradict REMS Team claims of constant sunny skies published for MSL at Gale Crater. Some MSSS reports are not as clear as we would like. Table 14 lists issues that need clarification.

But both rover sites experienced elevated atmospheric dust levels as a result of the storm, similar to atmospheric opacity levels experienced on typical hazy summer day in Los Angeles. With higher atmospheric dust concentrations came a warming of the thin Martian atmosphere, resulting in a diminishing of water ice cloud cover across the tropics.

Water ice clouds were present in the afternoon across the tropics of both hemispheres and over all the major shield volcanoes. Afternoon skies remained storm-free over the Curiosity rover site in Gale.

Since the landers are in the tropics we assume this means there were water ice clouds present. Does the statement that afternoon skies were storm free mean that there was no dust storms, but the skies were not sunny due to ice clouds?

Diffuse water ice clouds dominated the afternoon skies over all the major shield volcanoes, as well as, most tropical latitudes of both hemispheres. Skies were storm-free over Curiosity.

Since the landers are in the tropics we assume this means there were diffuse water ice clouds present. Does the statement that afternoon skies were storm free mean that there was no dust storms, but the skies were not sunny due to ice clouds?

Diffuse water ice clouds, associated with the developing aphelion cloud-belt, were present at equatorial latitudes and over the large shield volcanoes. Afternoon skies continued to remain storm-free over the Curiosity rover site in Gale Crater.

Since the landers are in the tropics we assume this means there were diffuse water ice clouds present. Does the statement that afternoon skies were storm free mean that there was no dust storms, but the skies were not sunny due to ice clouds?

The aphelion water ice cloud belt was present at equatorial latitudes. Skies were relatively clear and storm free over the Curiosity rover site in Gale Crater.

Since the landers are in equatorial latitudes we assume this means there were water ice clouds present. Does the statement that afternoon skies were relatively clear and storm free mean that there was no dust storms, but the skies were partly sunny rather than sunny?

Water ice clouds reaching altitudes of up to 30 kilometers, continued to be a prominent afternoon feature at tropical latitudes in both hemispheres. One of the first large dust storms of the Martian "regional dust storm season", covering an area greater than four times that of the state of California, began in Hellas Basin. During the next two sols the storm moved to the east, at an average speed of 25 m/s (about 56 mph). The western trailing edge of the storm was observed coming within 1440 kilometers of the Curiosity rover site. Though skies had become dustier over the last couple of months, both rover sites remained storm-free, at Endeavor and Gale crater.

Define the cut off between tropical latitudes (about 25 degrees North or South) and equatorial latitudes. If skies had become dustier over the last couple of months, but both rover sites remained storm-free, at Endeavor and Gale crater at what point does dust lower opacity from sunny to not sunny?

The regional storm in Promethei, noted in last week's report, had abated. A second storm developed early in week in Hesperia and moved north across the equator into Isidis in the northern hemisphere. However enough of that dust lofted into the atmosphere by the storm was transported eastward over the Curiosity rover site by the westerly (west-to-east) winds that dominate the tropical circulation. Both rover sites continued to remain storm-free, at Endeavor and Gale crater. The amount of dust transported was relatively small and had a negligible impact on rover operations and science.

If the storm persisted long enough that dust lofted into the atmosphere by the storm was transported eastward over the Curiosity rover site by the westerly (west-to-east) winds that dominate the tropical circulation, does this imply that it was not sunny?

Define "negligible" impact on rover operations and science. HOw does it differ from zero?

The widespread dust-lifting activity raised global atmospheric opacities to annual highs, as recorded by the Curiosity. While Curiosity experienced increased atmospheric opacities, it was largely spared from direct contact with storms. However Opportunity, just off to the east of the Acidalia storm-track, was less fortunate and experienced extremely hazy skies due to its proximity to areas of dust-lifting along the cross-equatorial storm track.

Last week on Mars, a local scale dust storm was observed originating out of western Elysium tracking southward towards Gale Crater. After reaching and partially obscuring Gale Crater, the storm quickly abated. Curiosity experienced elevated levels of atmospheric opacity during that time.

The condensate water-ice clouds strewn across the equatorial regions (the aphelion cloud-belt), continued to be the most prominent weather feature on Mars this past week. Storm-free skies persisted over Curiosity.

Were the condensate water-ice clouds strewn across the equatorial regions enough to keep the days from being sunny?

Curiosity in Gale Crater experienced scattered water ice cloud cover throughout the week, but remained free of any afternoon dust storm activity.

Curiosity experienced scattered water ice cloud cover throughout the week, but remained free of any afternoon dust storm activity. Ice clouds are different from dust. Was it or was it not sunny in the morning?

Figure 37 shows visibility for different values of opacity on Mars due to a dust storm at Opportunity between sols 1205 and 1235. All photos were taken between 10:53 and 11:30 local time. The dust in the Martian air over Opportunity blocked 99 percent of direct sunlight. This fact alone makes it very hard to accept that pressures would be unaffected.

D. Parsons (2000)80 addresses the compressibility of dust storms and positive feedback for their formation. Pre-dust storm density values are around 9.4 g/m3. A sample dust storm given in the Parsons paper would have additional densities of 17g/m3 in order to even be created. This is an order of magnitude greater than terrestrial storms. It also constitutes an increase of at least several hundred percent over previously accepted values. In the Sahara, pressures have been observed to increase during dust storms. Likewise when a huge dust storm hit Luke Air Force on July 5, 2011, pressure rose by 6.6 mbar (more than accepted average pressure at Mars areoid) between the storm’s arrival at 0255Z 6 July 2011 (pressure 1,004.7 mbar) and 0555Z when the pressure was up to 1,011.3 mbar. Pressure dropped as visibility cleared at 0655Z (personal call to Luke AFB meteorology, July 6, 2011).

The Parsons (2000)80 paper proposes a gravity current analog for dust storms and mentions that such currents should be constrained to the height of the inversion layer (but dust storms on Mars can still have effects at 160 km). Perhaps most important, increased pressure makes it easier to entrain particles (hence higher pressure may explain dust storms and dust devils).

Figure 38 is adapted from page 181 in The Martian Climate Revisited by Read and Lewis,79 which states that τ is derived from pressure data. During a Martian year opacity varies greatly. The clear season is in the northern summer with optical depth τ values of ~0.3 to 0.5. During northern winter τ values of ~2 to 5 or higher were seen during dust storms (see Figure 38). Black dots are the Year One data, black pluses are the Year Two data, and the red X’s are extrapolations from the pressure data. This is for Viking 1.